Refer to mount(8) for a description of generic mount options
available for all file systems. If you do not need to specify any
mount options, use the generic option defaults in /etc/fstab.
Options supported by all versions
These options are valid to use with any NFS version.
nfsvers=n
The NFS protocol version number used to contact the
server's NFS service. If the server does not support the
requested version, the mount request fails. If this
option is not specified, the client tries version 4.2
first, then negotiates down until it finds a version
supported by the server.
vers=n This option is an alternative to the nfsvers option. It
is included for compatibility with other operating systems
soft / hard
Determines the recovery behavior of the NFS client after
an NFS request times out. If neither option is specified
(or if the hard option is specified), NFS requests are
retried indefinitely. If the soft option is specified,
then the NFS client fails an NFS request after retrans
retransmissions have been sent, causing the NFS client to
return an error to the calling application.
NB: A so-called "soft" timeout can cause silent data
corruption in certain cases. As such, use the soft option
only when client responsiveness is more important than
data integrity. Using NFS over TCP or increasing the
value of the retrans option may mitigate some of the risks
of using the soft option.
softreval / nosoftreval
In cases where the NFS server is down, it may be useful to
allow the NFS client to continue to serve up paths and
attributes from cache after retrans attempts to revalidate
that cache have timed out. This may, for instance, be
helpful when trying to unmount a filesystem tree from a
server that is permanently down.
It is possible to combine softreval with the soft mount
option, in which case operations that cannot be served up
from cache will time out and return an error after retrans
attempts. The combination with the default hard mount
option implies those uncached operations will continue to
retry until a response is received from the server.
Note: the default mount option is nosoftreval which
disallows fallback to cache when revalidation fails, and
instead follows the behavior dictated by the hard or soft
mount option.
intr / nointr
This option is provided for backward compatibility. It is
ignored after kernel 2.6.25.
timeo=n
The time in deciseconds (tenths of a second) the NFS
client waits for a response before it retries an NFS
request.
For NFS over TCP the default timeo value is 600 (60
seconds). The NFS client performs linear backoff: After
each retransmission the timeout is increased by timeo up
to the maximum of 600 seconds.
However, for NFS over UDP, the client uses an adaptive
algorithm to estimate an appropriate timeout value for
frequently used request types (such as READ and WRITE
requests), but uses the timeo setting for infrequently
used request types (such as FSINFO requests). If the
timeo option is not specified, infrequently used request
types are retried after 1.1 seconds. After each
retransmission, the NFS client doubles the timeout for
that request, up to a maximum timeout length of 60
seconds.
retrans=n
The number of times the NFS client retries a request
before it attempts further recovery action. If the retrans
option is not specified, the NFS client tries each UDP
request three times and each TCP request twice.
The NFS client generates a "server not responding" message
after retrans retries, then attempts further recovery
(depending on whether the hard mount option is in effect).
rsize=n
The maximum number of bytes in each network READ request
that the NFS client can receive when reading data from a
file on an NFS server. The actual data payload size of
each NFS READ request is equal to or smaller than the
rsize setting. The largest read payload supported by the
Linux NFS client is 1,048,576 bytes (one megabyte).
The rsize value is a positive integral multiple of 1024.
Specified rsize values lower than 1024 are replaced with
4096; values larger than 1048576 are replaced with
1048576. If a specified value is within the supported
range but not a multiple of 1024, it is rounded down to
the nearest multiple of 1024.
If an rsize value is not specified, or if the specified
rsize value is larger than the maximum that either client
or server can support, the client and server negotiate the
largest rsize value that they can both support.
The rsize mount option as specified on the mount(8)
command line appears in the /etc/mtab file. However, the
effective rsize value negotiated by the client and server
is reported in the /proc/mounts file.
wsize=n
The maximum number of bytes per network WRITE request that
the NFS client can send when writing data to a file on an
NFS server. The actual data payload size of each NFS WRITE
request is equal to or smaller than the wsize setting. The
largest write payload supported by the Linux NFS client is
1,048,576 bytes (one megabyte).
Similar to rsize , the wsize value is a positive integral
multiple of 1024. Specified wsize values lower than 1024
are replaced with 4096; values larger than 1048576 are
replaced with 1048576. If a specified value is within the
supported range but not a multiple of 1024, it is rounded
down to the nearest multiple of 1024.
If a wsize value is not specified, or if the specified
wsize value is larger than the maximum that either client
or server can support, the client and server negotiate the
largest wsize value that they can both support.
The wsize mount option as specified on the mount(8)
command line appears in the /etc/mtab file. However, the
effective wsize value negotiated by the client and server
is reported in the /proc/mounts file.
ac / noac
Selects whether the client may cache file attributes. If
neither option is specified (or if ac is specified), the
client caches file attributes.
To improve performance, NFS clients cache file attributes.
Every few seconds, an NFS client checks the server's
version of each file's attributes for updates. Changes
that occur on the server in those small intervals remain
undetected until the client checks the server again. The
noac option prevents clients from caching file attributes
so that applications can more quickly detect file changes
on the server.
In addition to preventing the client from caching file
attributes, the noac option forces application writes to
become synchronous so that local changes to a file become
visible on the server immediately. That way, other
clients can quickly detect recent writes when they check
the file's attributes.
Using the noac option provides greater cache coherence
among NFS clients accessing the same files, but it
extracts a significant performance penalty. As such,
judicious use of file locking is encouraged instead. The
DATA AND METADATA COHERENCE section contains a detailed
discussion of these trade-offs.
acregmin=n
The minimum time (in seconds) that the NFS client caches
attributes of a regular file before it requests fresh
attribute information from a server. If this option is
not specified, the NFS client uses a 3-second minimum.
See the DATA AND METADATA COHERENCE section for a full
discussion of attribute caching.
acregmax=n
The maximum time (in seconds) that the NFS client caches
attributes of a regular file before it requests fresh
attribute information from a server. If this option is
not specified, the NFS client uses a 60-second maximum.
See the DATA AND METADATA COHERENCE section for a full
discussion of attribute caching.
acdirmin=n
The minimum time (in seconds) that the NFS client caches
attributes of a directory before it requests fresh
attribute information from a server. If this option is
not specified, the NFS client uses a 30-second minimum.
See the DATA AND METADATA COHERENCE section for a full
discussion of attribute caching.
acdirmax=n
The maximum time (in seconds) that the NFS client caches
attributes of a directory before it requests fresh
attribute information from a server. If this option is
not specified, the NFS client uses a 60-second maximum.
See the DATA AND METADATA COHERENCE section for a full
discussion of attribute caching.
actimeo=n
Using actimeo sets all of acregmin, acregmax, acdirmin,
and acdirmax to the same value. If this option is not
specified, the NFS client uses the defaults for each of
these options listed above.
bg / fg
Determines how the mount(8) command behaves if an attempt
to mount an export fails. The fg option causes mount(8)
to exit with an error status if any part of the mount
request times out or fails outright. This is called a
"foreground" mount, and is the default behavior if neither
the fg nor bg mount option is specified.
If the bg option is specified, a timeout or failure causes
the mount(8) command to fork a child which continues to
attempt to mount the export. The parent immediately
returns with a zero exit code. This is known as a
"background" mount.
If the local mount point directory is missing, the
mount(8) command acts as if the mount request timed out.
This permits nested NFS mounts specified in /etc/fstab to
proceed in any order during system initialization, even if
some NFS servers are not yet available. Alternatively
these issues can be addressed using an automounter (refer
to automount(8) for details).
nconnect=n
When using a connection oriented protocol such as TCP, it
may sometimes be advantageous to set up multiple
connections between the client and server. For instance,
if your clients and/or servers are equipped with multiple
network interface cards (NICs), using multiple connections
to spread the load may improve overall performance. In
such cases, the nconnect option allows the user to specify
the number of connections that should be established
between the client and server up to a limit of 16.
Note that the nconnect option may also be used by some
pNFS drivers to decide how many connections to set up to
the data servers.
rdirplus / nordirplus
Selects whether to use NFS v3 or v4 READDIRPLUS requests.
If this option is not specified, the NFS client uses
READDIRPLUS requests on NFS v3 or v4 mounts to read small
directories. Some applications perform better if the
client uses only READDIR requests for all directories.
retry=n
The number of minutes that the mount(8) command retries an
NFS mount operation in the foreground or background before
giving up. If this option is not specified, the default
value for foreground mounts is 2 minutes, and the default
value for background mounts is 10000 minutes (80 minutes
shy of one week). If a value of zero is specified, the
mount(8) command exits immediately after the first
failure.
Note that this only affects how many retries are made and
doesn't affect the delay caused by each retry. For UDP
each retry takes the time determined by the timeo and
retrans options, which by default will be about 7 seconds.
For TCP the default is 3 minutes, but system TCP
connection timeouts will sometimes limit the timeout of
each retransmission to around 2 minutes.
sec=flavors
A colon-separated list of one or more security flavors to
use for accessing files on the mounted export. If the
server does not support any of these flavors, the mount
operation fails. If sec= is not specified, the client
attempts to find a security flavor that both the client
and the server supports. Valid flavors are none, sys,
krb5, krb5i, and krb5p. Refer to the SECURITY
CONSIDERATIONS section for details.
sharecache / nosharecache
Determines how the client's data cache and attribute cache
are shared when mounting the same export more than once
concurrently. Using the same cache reduces memory
requirements on the client and presents identical file
contents to applications when the same remote file is
accessed via different mount points.
If neither option is specified, or if the sharecache
option is specified, then a single cache is used for all
mount points that access the same export. If the
nosharecache option is specified, then that mount point
gets a unique cache. Note that when data and attribute
caches are shared, the mount options from the first mount
point take effect for subsequent concurrent mounts of the
same export.
As of kernel 2.6.18, the behavior specified by
nosharecache is legacy caching behavior. This is
considered a data risk since multiple cached copies of the
same file on the same client can become out of sync
following a local update of one of the copies.
resvport / noresvport
Specifies whether the NFS client should use a privileged
source port when communicating with an NFS server for this
mount point. If this option is not specified, or the
resvport option is specified, the NFS client uses a
privileged source port. If the noresvport option is
specified, the NFS client uses a non-privileged source
port. This option is supported in kernels 2.6.28 and
later.
Using non-privileged source ports helps increase the
maximum number of NFS mount points allowed on a client,
but NFS servers must be configured to allow clients to
connect via non-privileged source ports.
Refer to the SECURITY CONSIDERATIONS section for important
details.
lookupcache=mode
Specifies how the kernel manages its cache of directory
entries for a given mount point. mode can be one of all,
none, pos, or positive. This option is supported in
kernels 2.6.28 and later.
The Linux NFS client caches the result of all NFS LOOKUP
requests. If the requested directory entry exists on the
server, the result is referred to as positive. If the
requested directory entry does not exist on the server,
the result is referred to as negative.
If this option is not specified, or if all is specified,
the client assumes both types of directory cache entries
are valid until their parent directory's cached attributes
expire.
If pos or positive is specified, the client assumes
positive entries are valid until their parent directory's
cached attributes expire, but always revalidates negative
entires before an application can use them.
If none is specified, the client revalidates both types of
directory cache entries before an application can use
them. This permits quick detection of files that were
created or removed by other clients, but can impact
application and server performance.
The DATA AND METADATA COHERENCE section contains a
detailed discussion of these trade-offs.
fsc / nofsc
Enable/Disables the cache of (read-only) data pages to the
local disk using the FS-Cache facility. See cachefilesd(8)
and <kernel_source>/Documentation/filesystems/caching for
detail on how to configure the FS-Cache facility. Default
value is nofsc.
sloppy The sloppy option is an alternative to specifying
mount.nfs -s option.
Options for NFS versions 2 and 3 only
Use these options, along with the options in the above
subsection, for NFS versions 2 and 3 only.
proto=netid
The netid determines the transport that is used to
communicate with the NFS server. Available options are
udp, udp6, tcp, tcp6, rdma, and rdma6. Those which end in
6 use IPv6 addresses and are only available if support for
TI-RPC is built in. Others use IPv4 addresses.
Each transport protocol uses different default retrans and
timeo settings. Refer to the description of these two
mount options for details.
In addition to controlling how the NFS client transmits
requests to the server, this mount option also controls
how the mount(8) command communicates with the server's
rpcbind and mountd services. Specifying a netid that uses
TCP forces all traffic from the mount(8) command and the
NFS client to use TCP. Specifying a netid that uses UDP
forces all traffic types to use UDP.
Before using NFS over UDP, refer to the TRANSPORT METHODS
section.
If the proto mount option is not specified, the mount(8)
command discovers which protocols the server supports and
chooses an appropriate transport for each service. Refer
to the TRANSPORT METHODS section for more details.
udp The udp option is an alternative to specifying proto=udp.
It is included for compatibility with other operating
systems.
Before using NFS over UDP, refer to the TRANSPORT METHODS
section.
tcp The tcp option is an alternative to specifying proto=tcp.
It is included for compatibility with other operating
systems.
rdma The rdma option is an alternative to specifying
proto=rdma.
port=n The numeric value of the server's NFS service port. If
the server's NFS service is not available on the specified
port, the mount request fails.
If this option is not specified, or if the specified port
value is 0, then the NFS client uses the NFS service port
number advertised by the server's rpcbind service. The
mount request fails if the server's rpcbind service is not
available, the server's NFS service is not registered with
its rpcbind service, or the server's NFS service is not
available on the advertised port.
mountport=n
The numeric value of the server's mountd port. If the
server's mountd service is not available on the specified
port, the mount request fails.
If this option is not specified, or if the specified port
value is 0, then the mount(8) command uses the mountd
service port number advertised by the server's rpcbind
service. The mount request fails if the server's rpcbind
service is not available, the server's mountd service is
not registered with its rpcbind service, or the server's
mountd service is not available on the advertised port.
This option can be used when mounting an NFS server
through a firewall that blocks the rpcbind protocol.
mountproto=netid
The transport the NFS client uses to transmit requests to
the NFS server's mountd service when performing this mount
request, and when later unmounting this mount point.
netid may be one of udp, and tcp which use IPv4 address
or, if TI-RPC is built into the mount.nfs command, udp6,
and tcp6 which use IPv6 addresses.
This option can be used when mounting an NFS server
through a firewall that blocks a particular transport.
When used in combination with the proto option, different
transports for mountd requests and NFS requests can be
specified. If the server's mountd service is not
available via the specified transport, the mount request
fails.
Refer to the TRANSPORT METHODS section for more on how the
mountproto mount option interacts with the proto mount
option.
mounthost=name
The hostname of the host running mountd. If this option
is not specified, the mount(8) command assumes that the
mountd service runs on the same host as the NFS service.
mountvers=n
The RPC version number used to contact the server's
mountd. If this option is not specified, the client uses
a version number appropriate to the requested NFS version.
This option is useful when multiple NFS services are
running on the same remote server host.
namlen=n
The maximum length of a pathname component on this mount.
If this option is not specified, the maximum length is
negotiated with the server. In most cases, this maximum
length is 255 characters.
Some early versions of NFS did not support this
negotiation. Using this option ensures that pathconf(3)
reports the proper maximum component length to
applications in such cases.
lock / nolock
Selects whether to use the NLM sideband protocol to lock
files on the server. If neither option is specified (or
if lock is specified), NLM locking is used for this mount
point. When using the nolock option, applications can
lock files, but such locks provide exclusion only against
other applications running on the same client. Remote
applications are not affected by these locks.
NLM locking must be disabled with the nolock option when
using NFS to mount /var because /var contains files used
by the NLM implementation on Linux. Using the nolock
option is also required when mounting exports on NFS
servers that do not support the NLM protocol.
cto / nocto
Selects whether to use close-to-open cache coherence
semantics. If neither option is specified (or if cto is
specified), the client uses close-to-open cache coherence
semantics. If the nocto option is specified, the client
uses a non-standard heuristic to determine when files on
the server have changed.
Using the nocto option may improve performance for read-
only mounts, but should be used only if the data on the
server changes only occasionally. The DATA AND METADATA
COHERENCE section discusses the behavior of this option in
more detail.
acl / noacl
Selects whether to use the NFSACL sideband protocol on
this mount point. The NFSACL sideband protocol is a
proprietary protocol implemented in Solaris that manages
Access Control Lists. NFSACL was never made a standard
part of the NFS protocol specification.
If neither acl nor noacl option is specified, the NFS
client negotiates with the server to see if the NFSACL
protocol is supported, and uses it if the server supports
it. Disabling the NFSACL sideband protocol may be
necessary if the negotiation causes problems on the client
or server. Refer to the SECURITY CONSIDERATIONS section
for more details.
local_lock=mechanism
Specifies whether to use local locking for any or both of
the flock and the POSIX locking mechanisms. mechanism can
be one of all, flock, posix, or none. This option is
supported in kernels 2.6.37 and later.
The Linux NFS client provides a way to make locks local.
This means, the applications can lock files, but such
locks provide exclusion only against other applications
running on the same client. Remote applications are not
affected by these locks.
If this option is not specified, or if none is specified,
the client assumes that the locks are not local.
If all is specified, the client assumes that both flock
and POSIX locks are local.
If flock is specified, the client assumes that only flock
locks are local and uses NLM sideband protocol to lock
files when POSIX locks are used.
If posix is specified, the client assumes that POSIX locks
are local and uses NLM sideband protocol to lock files
when flock locks are used.
To support legacy flock behavior similar to that of NFS
clients < 2.6.12, use 'local_lock=flock'. This option is
required when exporting NFS mounts via Samba as Samba maps
Windows share mode locks as flock. Since NFS clients >
2.6.12 implement flock by emulating POSIX locks, this will
result in conflicting locks.
NOTE: When used together, the 'local_lock' mount option
will be overridden by 'nolock'/'lock' mount option.
Options for NFS version 4 only
Use these options, along with the options in the first subsection
above, for NFS version 4.0 and newer.
proto=netid
The netid determines the transport that is used to
communicate with the NFS server. Supported options are
tcp, tcp6, rdma, and rdma6. tcp6 use IPv6 addresses and
is only available if support for TI-RPC is built in. Both
others use IPv4 addresses.
All NFS version 4 servers are required to support TCP, so
if this mount option is not specified, the NFS version 4
client uses the TCP protocol. Refer to the TRANSPORT
METHODS section for more details.
minorversion=n
Specifies the protocol minor version number. NFSv4
introduces "minor versioning," where NFS protocol
enhancements can be introduced without bumping the NFS
protocol version number. Before kernel 2.6.38, the minor
version is always zero, and this option is not recognized.
After this kernel, specifying "minorversion=1" enables a
number of advanced features, such as NFSv4 sessions.
Recent kernels allow the minor version to be specified
using the vers= option. For example, specifying vers=4.1
is the same as specifying vers=4,minorversion=1.
port=n The numeric value of the server's NFS service port. If
the server's NFS service is not available on the specified
port, the mount request fails.
If this mount option is not specified, the NFS client uses
the standard NFS port number of 2049 without first
checking the server's rpcbind service. This allows an NFS
version 4 client to contact an NFS version 4 server
through a firewall that may block rpcbind requests.
If the specified port value is 0, then the NFS client uses
the NFS service port number advertised by the server's
rpcbind service. The mount request fails if the server's
rpcbind service is not available, the server's NFS service
is not registered with its rpcbind service, or the
server's NFS service is not available on the advertised
port.
cto / nocto
Selects whether to use close-to-open cache coherence
semantics for NFS directories on this mount point. If
neither cto nor nocto is specified, the default is to use
close-to-open cache coherence semantics for directories.
File data caching behavior is not affected by this option.
The DATA AND METADATA COHERENCE section discusses the
behavior of this option in more detail.
clientaddr=n.n.n.n
clientaddr=n:n:...:n
Specifies a single IPv4 address (in dotted-quad form), or
a non-link-local IPv6 address, that the NFS client
advertises to allow servers to perform NFS version 4.0
callback requests against files on this mount point. If
the server is unable to establish callback connections to
clients, performance may degrade, or accesses to files may
temporarily hang. Can specify a value of IPv4_ANY
(0.0.0.0) or equivalent IPv6 any address which will signal
to the NFS server that this NFS client does not want
delegations.
If this option is not specified, the mount(8) command
attempts to discover an appropriate callback address
automatically. The automatic discovery process is not
perfect, however. In the presence of multiple client
network interfaces, special routing policies, or atypical
network topologies, the exact address to use for callbacks
may be nontrivial to determine.
NFS protocol versions 4.1 and 4.2 use the client-
established TCP connection for callback requests, so do
not require the server to connect to the client. This
option is therefore only affect NFS version 4.0 mounts.
migration / nomigration
Selects whether the client uses an identification string
that is compatible with NFSv4 Transparent State Migration
(TSM). If the mounted server supports NFSv4 migration
with TSM, specify the migration option.
Some server features misbehave in the face of a migration-
compatible identification string. The nomigration option
retains the use of a traditional client indentification
string which is compatible with legacy NFS servers. This
is also the behavior if neither option is specified. A
client's open and lock state cannot be migrated
transparently when it identifies itself via a traditional
identification string.
This mount option has no effect with NFSv4 minor versions
newer than zero, which always use TSM-compatible client
identification strings.
